Technical Field
The present invention relates to flash memory devices, and in particular to methods for migrating data to avoid read disturbance and apparatuses using the same.
Description of the Related Art
Flash memory devices typically include NOR flash devices and NAND flash devices. NOR flash devices are random access—a host accessing a NOR flash device can provide the device with any address on its address pins and immediately retrieve data stored in that address on the device's data pins. NAND flash devices, on the other hand, are not random access but serial access. It is not possible for NOR to access any random address in the way described above. Instead, the host has to write into the device a sequence of bytes which identifies both the type of command requested (e.g. read, write, erase, etc.) and the address to be used for that command. The address identifies a page (the smallest chunk of flash memory that can be written in a single operation) or a block (the smallest chunk of flash memory that can be erased in a single operation), and not a single byte or word. In reality, the NAND flash device always reads complete pages from the memory cells and writes complete pages to the memory cells. After a page of data is read from the array into a buffer inside the device, the host can access the data bytes or words one-by-one by serially clocking them out using a strobe signal.
Reading NAND flash memory may cause nearby physical blocks (or wordlines) to change over time (i.e. to become programmed). If one physical block is read from continually, that physical block will not fail but rather the surrounding physical block or blocks on subsequent reads will fail. The threshold number of reads between intervening erase operations is generally in the thousands of reads. This is known as read disturb. Accordingly, what is needed are methods for migrating data to avoid read disturbance and apparatuses using the same.